OAuth: Add support for empty scopes (#32129)

* add parameter empty_scopes to override scope parameter with empty value and thus be able to authenticate against IdPs without scopes. Issue #27503

Update docs/sources/auth/generic-oauth.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* updated check according to feedback

* Update generic-oauth.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

pkg/login/social/social.go

@@ -85,6 +85,7 @@ func NewOAuthService() {
 
 	for _, name := range allOauthes {
 		sec := setting.Raw.Section("auth." + name)
+
 		info := &setting.OAuthInfo{
 			ClientId:            sec.Key("client_id").String(),
 			ClientSecret:        sec.Key("client_secret").String(),
@@ -107,6 +108,11 @@ func NewOAuthService() {
 			TlsSkipVerify:       sec.Key("tls_skip_verify_insecure").MustBool(),
 		}
 
+		// when empty_scopes parameter exists and is true, overwrite scope with empty value
+		if sec.Key("empty_scopes").MustBool() {
+			info.Scopes = []string{}
+		}
+
 		if !info.Enabled {
 			continue
 		}